The invention relates to an arrangement comprising a thermo-electric generator having a hot side which absorbs heat from a heat source, a cold side which discharges heat to a heat sink, and electrical terminals for outputting electrical energy with an output voltage and an electric circuit with a maximum permissible input voltage, the inputs of which are connected to the electrical terminals of the thermo-electric generator. Furthermore, the invention relates to a method for operating such an arrangement.
A thermo-electric generator, also referred to for short as TEG, is a device which converts heat energy into electrical energy using the thermo-electric effect.
The thermo-electric effect, also referred to as the Seebeck effect, describes the reversible interaction between temperature and electricity. The Seebeck voltage is determined byUSeebeck=α×δT where                δT is the temperature difference between the hot and cold sides        α—is the Seebeck coefficient or thermo-electric force.        
The Seebeck coefficient has the dimension of an electric voltage per temperature difference (V/K). The value of the Seebeck coefficient is responsible to a decisive degree for the magnitude of the Seebeck voltage.
A thermo-electric generator is composed of differently doped semiconductor materials. Customary semiconductor materials for thermo-electric generators are semiconductor materials such as in particular, Bi2Te3, Bi2Se, Sb2Te3, PbTe, SiGe or FeSi2 and the alloys thereof.
A conventional thermo-electric generator is constructed from two or more small squares which are each made of p-type doped and n-type doped semiconductor material and are alternately connected to one another at the top and the bottom by metal bridges. The metal bridges form at the same time the thermal contact faces and are usually insulated by a ceramic plate. The different squares made of p-type and n-type domed doped semiconductor material are connected electrically in series. The ceramic plates are at a distance of approximately 3 mm to 5 mm from each other and the squares are arranged, in particular soldered into place between them. One of the ceramic plates forms the hot side and the ceramic plate lying opposite forms the cold side, of the thermo-electric generator. The hot side absorbs heat from a heat source, while the cold side discharges heat to a heat sink. Bringing about the temperature difference δT between the hot and cold sides causes the Seebeck voltage USeebeck to be generated at the terminals of the thermo-electric generator.
EP 1 287 566 B1 discloses a thermo-electric element and a module with a plurality of thermo-electric elements connected electrically in series, in order to increase the efficiency of a thermo-electric generator. The thermo-electric element has at least one n-type layer and at least one p-type layer of one or more doped semiconductors, wherein the n-type layer/layers and the p-type layer/layers are arranged by forming at least one pn junction. At least one n-type layer and at least one p-type layer are placed in electrically selective contact and a temperature gradient is applied in parallel (x direction) with the boundary layer, between at least one n-type layer and one p-type layer. At least one pn junction is formed essentially along the entire, preferably longest, extent of the n-type layer/layers and the layer/layers, and therefore essentially along the entire boundary layer thereof.
As a result of the temperature gradient along the large-area pn boundary face there is a temperature difference along this elongate pn transition between two ends of a pn layer assembly which causes the efficiency of the thermo-electric element to be higher than in conventional thermo-electric generators which do not have a temperature gradient along and within the pn junction. The thermo-electric elements are arranged thermally in parallel between two plates in the module. The plates serve to bring about improved thermal coupling both on the cold and hot sides. They are preferably embodied as good thermal conductors and are, in particular, composed of ceramic, electrically non-conductive materials. The disclosure of EP 1 287 566 B1, in particular with respect to the structure of the thermo-electric element (FIG. 3) and of the module (FIG. 13) as well as with respect to the semiconductor materials used, is included expressly in the present application.
An arrangement of the generic type is disclosed in DE 10 2008 023 806 A1. The arrangement is integrated into the exhaust system of a motor vehicle in which what is referred to as the hot side of the TEG is connected in a thermally conductive fashion to an exhaust-gas-conducting line of the exhaust section, while the cold side of the TEG is thermally coupled, for example, to a coolant line, conducting a coolant, of the engine cooling system of the motor vehicle. The TEG is connected electrically into the on-board electrical system of the motor vehicle via a direct voltage connection device in the form of a direct voltage transformer. The arrangement composed of the TEG and the direct voltage transformer improves the energetic efficiency of the motor vehicle considerably. However, the integration of the arrangement into the exhaust system of the motor vehicle requires a redesign of the exhaust system. The exhaust system comprises an exhaust duct which has two component exhaust ducts running in parallel, wherein the component exhaust ducts are combined again downstream. One of the two component exhaust ducts is thermally coupled to the thermo-electric generator, wherein at least switching element for directing the stream of exhaust gas is present in the exhaust duct in such a way that depending on the switched position of the switching element the stream of exhaust gas flows exclusively through the first component exhaust duct, exclusively through the second component exhaust duct or proportionally through both component exhaust ducts. In addition, a control device is provided for actuating the at least one switching element. The output voltage of the thermo-electric generator which is arranged in the component exhaust duct is virtually proportional to the temperature difference between the hot and cold sides of the thermo-electric generator. In order to avoid damage to the direct voltage transformer which is connected to the thermo-electric generator and to the on-board electrical, system of the motor vehicle by an excessively high output voltage of the thermo-electric generator in certain operating situations, it is necessary to conduct hot exhaust gases past the thermo-electric generator. This bypass solution makes it possible to configure the thermo-electric generator for medium motor power levels and therefore exhaust gas temperatures and exhaust gas mass flow rates which make up the greater part of the driving cycle. Furthermore, the direct voltage transformer can be configured for the medium power range which is used most frequently. However, a significant disadvantage of the prior art is that the exhaust system has to have two component exhaust ducts running in parallel and in addition controlled switching elements have to be installed in the exhaust section.